The present invention relates to transferring data from a packet network to a digital radio network whose transmission channel enables transmission of audio and data services and selective reception of these services.
A known packet relay mechanism is represented by Frame Relay FR. In FR transfer technique, error correction and flow control are carried out at termination points of the network and not in every link as in packet data transfer according to the X.25 standard. FR speeds up routing of packets via several connection points to their destination, because it is not necessary to check, as far as errors are concerned, every packet received before transferring them to the next connection point. Originally, FR was intended for interconnecting private data networks, such as LAN networks, and it therefore defines the interface for the high-rate data between the FR network and the computers of the user. The routing is based on a local address conveyed in the frame. The frame structure per se is specified in the recommendation CCITT I.441, and parameters related to the protocol are specified in the recommendations I.233 and Q.922. The significant matter as far as the method of transfer is concerned is that the frame length is not constant, but the transfer participants negotiate the frame length to be used prior to the transfer.
No fixed bandwidth will be allocated for the service, but specific service parameters relevant to the nature of the service will be determined. These include committed information rate CIR, committed burst size Bc and excess burst size Be.
FIG. 1A shows the structure of an FR frame. The frame comprises five fields. A frame begins and ends with a flag one octet long which is a specified bit sequence containing six successive 1 bits. For logical channel numbering, an address field is used, the contents of which will be disclosed below. An information field succeeding the address field does not have a fixed length, but contains an even number of octets of user data. The user and the FR network on the one hand, and the FR networks on the other hand, negotiate with one another the data field length to be used, whereby the frame length may be up to 4096 octets. The information field is followed by a frame check sequence FCS, two octets in length.
FIG. 1B shows the contents of an FR frame address field. The field contains address field extension bits EA, a C/R (Command/Response) identifier bit allocated for use by the end-user's equipment, forward and backward explicit congestion indicator bits FECN and BECN on the basis of which the system is able to reduce traffic rate, a discard eligibility indicator DE which means that frames containing this indicator can under specific conditions be discarded, and, essential to the present invention, a permanent virtual circuit or a data link connection identifier DLCI. The DLCI thus indicates at each nodal point of the network where a packet is to be routed. The minimum length of the address field is two octets as in FIG. 1B, but it can be prolonged to be three or four octets in length, as illustrated in FIGS. 1C and 1D. By employing two or three octets in the address field, it is possible to support a wider DLCI address space between the user and the network, or within the network.
Relating to a wideband ISDN network, an asynchronous transfer mode ATM has been developed. The ATM outlines data transfer in a packet or circuit-switched fixed network. The data are transferred in fixed-length 53-byte ATM cells, each cell having five bytes as header of the cell and the remaining 48 bytes as actual information. The ATM cells are specified in the recommendations CCITT Recommendation I.361 and CCITT Draft Recommendation I.150. In simplified terms, it can be noted that the user information to be transferred is split into segments of fixed length, and each segment is inserted in the information field of the ATM cell. The number of segments in a unit time represents the transfer capacity required by the user. In addition, a header, to be disclosed in greater detail below, will be inserted in the information field, whereby a fixed-length 53-byte ATM cell will be created. A cell is an independent data transfer unit as it contains information on the receiving party's address on the basis of which the receiving party can be located in the network.
An ATM network is flexible, fast and suitable for different kind of services particularly due to its small cell size. A service may flexibly reserve capacity according to its needs by reserving an appropriate number of cells.
FIGS. 2A and 2B illustrate the structure of both the cell and different header fields. The header structure employed depends on what segment in the ATM network is under examination. The cell structure illustrated in FIG. 2A is only employed in a user-network interface UNI, and the header structure of FIG. 2B is employed in the rest of the ATM network, i.e. in a network-node interface NNI. In the header of the ATM cell illustrated in FIG. 2A, the field GFR (Generic Flow Control) is intended for traffic control carried out in UNI. The other fields are the same as in the header of the cell in FIG. 2B, and they are PT (Payload Type) field which distinguishes the network control cells from the subscriber's cells, an RES (Reserved) field, which is in reserve, a CLP (Cell Loss Priority) field which assigns a priority to the destruction of cells, and an HEC (Header Error Control) field which is the header checksum.
As far as the present invention is concerned, the most important part of the header is a routing field. It consists of two parts: virtual path identifier VPI, and virtual channel identifier VCI. At the user-network interface UNI, the identifier VPI consists of 8 bits, and at the network-node interface NNI this identifier consists of 12 bits. The virtual channel identifier VCI has 16 bits at both the interfaces. As its name indicates, the routing field acts as the basis for routing cells in the ATM network. The VPI is primarily used in the internal parts of the network, and in practice it often determines to which physical connection the cell is to be routed. Instead, the VCI is often used for routing purposes at the network border only, e.g. where the user's PC is coupled to the ATM network. However, it should be noted that the route for the cell is only determined by the VPI and the VCI together.
In the above, two packet networks operating at the fixed network side have been described. In the following, a brief description will be given of the new DAB (Digital Audio Broadcasting) system, developed for efficient use of radio frequency bands. In particular, DAB has been developed for transmitting audio and data services to a mobile environment, i.e. to a mobile receiver.
DAB specifies a digital radio channel based on several carriers. The transmission channel may be either a continuous data stream channel or a packet channel; however, packet transfer is more flexible and in an easier way enables the transfer of data units having a finite length. From the point of view of transmission, transfer and reception, a thorough description of the DAB system is given e.g. in "A channel Encoder/Decoder for DAB Demonstrator", MSc Thesis, 1995, Kimmo Hakkarainen, Tampere University of Technology, pp. 2-13.
Reference is now made to FIG. 3, illustrating a DAB system in a simplified form. At the transmitting end, a transmission control unit 1 controls the transmission. An FIC and control block 2 generates SI (Service Information), concerning the audio and data services, MCI (Multiplex Configuration Information), and CA information (Conditional Access) which may concern the charging/encryption of the services. Together, these form a FIC (Fast Information Channel). Audio information, e.g. music, provided by audio services providers 3, is compressed in an encoder 4 and applied to audio channel encoders 5. Correspondingly, data supplied by providers of data services are coded in data channel encoders 7. The channel coded data, audio and FIC information are applied to block 8 carrying out OFDM (Orthogonal Frequency Division Multiplexing). An OFDM symbol generated by the block as a reverse fast Fourier transformation is a group of sub-carriers, and it has a precisely defined duration. Single sub-carriers are modulated by D-QPSK method (Differential Quaternary Phase Shift Keying), and eventually a DAB transmission signal will be obtained which is comprised of successive transfer frames. Therefore, each frame is time-multiplexed between the synchronization channel, fast information channel FIC, and an MSC (Main Service Channel) containing the audio and data services. At the receiving end, the signal is decoded in a COFDM (Coded Orthogonal Frequency Division Multiplex) block 9, which converts the I-Q signal into digital form, the digitized signal is converted into the frequency domain by a fast Fourier transformation, the frequency interleaving is removed, and transfer frames are made up from successive OFDM symbols. The information channel FIC and the MSC channel containing the audio and data services are separated, and sub-channels are separated from the MSC channel and they are channel decoded in decoders 5' and 6'. Desired sub-channels are then applied to further processing. On the basis of the FIC channel received, the subscriber will know the services contained in the signal received, and may accordingly select the service or services he desires.
An advantage of the DAB system is that the service providers can be allocated data capacity dynamically. The capacity may instantaneously reach 1.728 Mbit/s, maximum. In such a case, the data are transmitted in packets according to FIG. 4A, consisting of a header field, data field and checksum. The significance of the fields described is in accordance with the DAB standard. The packet header contains information of the packet length (PKT LEN) which may be 24, 48, 72 or 96 bytes, continuity index (CONT IND), first/last packet information (FIRST/LAST), address identifying the service component (PKT ADDRESS), a command (Command), and the length of the actual data field (DATA LEN). A data field (DATA FIELD) contains the actual data conveyed and, if necessary, padding bits. Finally, a packet checksum (PKT CRC) is inserted.
The packets are made up of a Data Group according to FIG. 4B. The Data Group, in turn, is formed so that the data stream provided by the data service provider of e.g. FIG. 6 from an information source is divided into bytes and the bytes are used for filling the data field of the Data Group. A group header and a session header are added. As soon as the data field of the Data group is full, the entire Data group will be divided mechanically into fixed-length segments without paying attention to the information or fields. Each segment constitutes the data field segment of the data packet. When a header is added to each of the fixed-length segments obtained by dividing, the result is a data packet according to FIG. 4A, which will be transferred on the transmission path.
Usually, the Data Group consists of data fields of several packets transmitted successively. In its simplest form, one packet is enough to form a Data Group.
The Data Group fields are illustrated in FIG. 5. The fields of the data group header are: EXT FL=Extension flag, CRC FL=CRC flag, SES FL=Session Flag, DG TYPE=Data Group type, CONT IND=Continuity index, REP IND=Repetition index, and EXT FIELD=Extension field.
The fields of the session header are LAST FL=Last, SEG NUM=Segment number, RFA=Reserved, LEN IND indicates the length of the succeeding address field and ADDR FIELD=the address of the end-user.
These header fields are followed by the actual data, and the checksum DG CRC of the Data Group. By means of the end-user's address ADDR FIELD, the packets are directed to the receiving party indicated in the reception, who is then able to utilize the service conveyed in the packets.
As indicated by FIG. 3 and the description related thereto, the idea behind the DAB system is that the units 3 and 4 of the audio and data services providers are directly connected to the transmitter in a similar manner as in present broadcasts which in most cases have the programme source in a studio or at the end of a direct link connection. On the other hand, there are presently a large number of service providers, such as those offering e.g. library services and various kinds of information services, via X.25, X.400, Internet, etc. connections. All the services can be transferred via the ATM or FR network. It would be natural for the service provider to establish his service via the ATM or FR network to be a DAB service source, whereby the service could be conveyed by means of the DAB system to a mobile receiving party and/or group of subscribers.
The above is associated with the problem that no mechanism exists by means of which a digital packet network, especially an ATM or an FR network, can be connected to the DAB system.